BEIJING, April 22 -- Chinese cities reported more days with sound air quality on average in March compared with a year ago, the Ministry of Environmental Protection said on Wednesday.

In a monthly report, the ministry said the 74 cities under its watch reported air pollution on 28.9 percent of the days last month, down from 37.7 percent in March 2013.

Though Beijing and its 12 neighboring cities in north China's Beijing-Tianjin-Hebei region also saw a slight increase in days of sound air quality, the proportion of their smoggy days more than doubled the country's average.

The chief pollutant in the region was PM2.5, followed by PM10.

The ministry said seven of the 10 Chinese cities reporting the worst air quality last month were in Hebei Province, which neighbors Beijing. The other three cities were Zhengzhou, Jinan and Shenyang.

China began to include PM2.5, a key indicator of air pollution, and ozone in its new air quality standard in 2013.